An ultraviolet curable resin composition can be cured to form a cured product due to the irradiation of ultraviolet rays without a heating process in general. Accordingly, in recent years, an ultraviolet curable resin composition has been noted as a material, for example, which can be developed into use for a surface coating, an adhesive and the like for electronic members, plastics base materials and the like, which are easily damaged by heating.
The ultraviolet curable resin composition is roughly classified into a radically polymerizable resin composition and an ionic polymerizable resin composition.
As the radically polymerizable ultraviolet curable resin composition, a resin composition which contains a compound having a polymerizable unsaturated double bond and a radical polymerization initiator, for example, has been known.
However, when a radical polymerization of the aforementioned ultraviolet curable resin composition is conducted, the curing of the resin composition may stop on occasion before the curing proceeds sufficiently because of a cessation reaction between radicals or radical deactivation caused by the influence of oxygen in the atmosphere.
On the other hand, as the ionic polymerizable ultraviolet curable resin composition, it is known that there are a cationic-polymerizable resin composition, wherein active species at the time of polymerization is a cation, and an anionic-polymerizable resin composition, wherein active species at the time of polymerization is an anion.
The ionic polymerization is not a curing reaction which is caused by a radical. Accordingly, neither the aforementioned cessation reaction between radicals nor the radical deactivation are caused, and the ionic polymerization can proceed successively even when exposure to ultraviolet rays is stopped in the middle of a polymerization of an ionic polymerizable ultraviolet curable composition.
Therefore, the ionic-polymerizable ultraviolet curable resin composition is expected to be used as a material which can be adopted in a case wherein continuous exposure to ultraviolet rays which are required for almost complete polymerization of a resin composition is difficult. For example, the ionic-polymerizable ultraviolet curable resin composition is expected to be used as an adhesive material usable for adhering an opaque base material.
As the ionic-polymerizable ultraviolet curable resin composition, such a composition which includes a urethane resin and a photopolymerization initiator has been examined. Although the degree thereof varies in accordance with the composition of each urethane resin, many urethane resins have a level of flexibility such that, for example, when such a urethane resin is used for an adhesive, the adhesive can be deformed to some extent in accordance with deformation of a substrate adhered to by the adhesive.
Therefore, if it becomes possible to cure a urethane resin by ionic polymerization, it is possible to provide an adhesive, a coating and the like which have high added value such that a cured product, excellent in flexibility and the like, are formed without a heating step.
As the ionic-polymerizable ultraviolet curable resin composition including a urethane resin, for example, a cationic-photopolymerizable resin composition has been known which comprises; a polyurethane resin having an oxetanyl group at the terminal end thereof; an acrylic resin having a hydroxyl group and an alicyclic epoxy group as the side chain thereof; a compound having an oxetanyl group; an alicyclic epoxy resin; and a cationic-polymerization initiator (for examples, refer to the Patent Document 1).
However, it is hard to say that the aforementioned ionic-polymerizable ultraviolet curable resin composition has sufficient curability for practical use, since there is a case that a cationic polymerization of the resin composition cannot proceed sufficiently even when ultraviolet rays are irradiated in an amount that it is sufficient in general. Such a cured product wherein a cationic polymerization proceeds insufficiently has problems such that the cured product is easily eroded by organic solvents or the like.
As described above, such an ultraviolet curable resin composition including a urethane resin has not yet been found wherein a cationic polymerization thereof can fully proceed by ultraviolet exposure, and the ultraviolet curable resin composition can form a cured product which has excellent flexibility on a level which can follow modification, such as a bend or the like, of various substrates.
Patent Document 1: Japanese Unexamined Patent Application Publication No. 2001-246706